Comment on “Super-adsorbent hydrogel for removal of methylene blue dye from aqueous solution” by X.-S. Hu, R. Liang and G. Sun, J. Mater. Chem. A, 2018, 6, 17612–17624

Abstract

This work aims to present several concerns and additional arguments related to an original paper previously published in the journal. Firstly, there is not enough evidence to convince us that the chemical adsorption of methylene blue dye by the hydrogel was only based on the best fitting of the experimental data of time-dependent adsorption to the pseudo-second-order model. Secondly, the adsorption isotherm needs to be correctly presented as the plot of q_eversus C_e (not C_o). The application of q_eversus C_o for calculating the parameters of the adsorption isotherm models (i.e., Langmuir and Freundlich) led to errors in the results of the magnitude of those parameters. From the adsorption isotherm (reported by the original authors), it is concluded that they forget to test the blank samples (denoted as C_o values in the adsorption studies). The blank samples always play an important role in obtaining the correct calculation of the adsorption capacity (q_e) of adsorbents towards adsorbate. Thirdly, the technical term of the isoelectric point (pH_IEP) (not the point of zero charge, pH_PZC) needs to be used when the electrical state of the adsorbent’s surface in solution was determined by a zeta analyzer. Some other comments and concerns (i.e., the method to determine the hydrogel’s porosity using the Adobe Photoshop program, the dried mass of the hydrogel, the unit of the Freundlich constant K_F, and the appropriate selection of initial dye concentrations for studying the cycles of adsorption and desorption) are also discussed herein. The author hopes that these comments will helpful for other researchers and the original authors to maintain the high quality of publication in the field of adsorption science and technology in the future.